1. Technical Field
The invention relates to plasma generation, particularly to plasma generators built in refrigerators.
2. Related Art
Refrigerators are a necessary appliance in daily life. A refrigerator is a hodgepodge of all kinds of fresh foods, so it is always full of odor and bacteria. Thus, many refrigerators are equipped with a deodorizer and/or sterilizer, such as silver ion, ultraviolet (UV), ozone, enzyme or negative ion. All these deodorizers and sterilizers are under the reach of plasma in performance. On the other hand, vegetables and fruit will produce ethylene during storage in a refrigerator. Ethylene is a gaseous plant hormone, which can regulate or affect the physiological process of growth and development of plants. As a result, effectively reducing production of ethylene can defer fruit's maturation. Plasma possesses a great effect in decomposition and reduction of ethylene.
Plasma is a gas which is dissociated into positive and negative particles in an electromagnetic field. Plasma contains negative and positive ions, electrons, free radical and neutral particles. In an electric field, electrons are much larger than positive ions in mean free path, so electrons can be intensively accelerated to carry high energy (10 or more eV). Such high-energy electrons colliding with atoms or molecules can break chemical bonds in a molecule to form reactive free radical or to ionize the atoms and molecules. The ionized electrons will be further accelerated to make other collisions and to form more electrons, ions and free radicals (called “chain reaction”). Finally, plasma with high-energy electrons and highly-reactive free radicals is produced. U.S. Pat. Nos. 3,212,974, 4,275,287 and 4,778,561 disclose methods for producing plasma.
In an electric field, electrons in plasma are much easier to absorb energy than ions. Thus temperature of the electrons in plasma can reach up to hundreds of thousands of centigrade and that of the other particles is about room temperature. Because high-temperatured electrons are high reactive and can easily break bonds, plasma can be obtained at room temperature.
It is the most effective and economical manner to produce plasma at 1 atm. In order to stably produce plasma, the system must operate at low pressure. Thus a vacuum chamber and vacuum pump are needed to keep a low pressure circumstance. This will increase costs of manufacturing and maintenance and decrease the amount of production in a unit time. For example, a vacuum pump is easy to be damaged by particles and corrosion. A vacuum pump is not needed if plasma is produced at normal atmospheric pressure. Atmospheric pressure plasma technology can simplify manufacturing costs and process. As a result, atmospheric pressure plasma technology can extensively extend application of plasma.
Electrons in low pressure plasma are much longer than other particles in mean free path, so it can obtain more energy enough to ionize other gases. When the pressure is 1 atm., however, gaseous molecules collide frequently and the mean free path is so short. It is hard that electrons obtain enough energy to ionize gaseous molecules. As a result, plasma cannot be produced.
At this time, there are two methods for producing plasma:
(1) increasing the voltage of the external power source to thousands of volts to add energy to electrons; and (2) providing a large amount of current to heat up the gaseous molecules to form high temperature plasma.
Additionally, in comparison with low pressure plasma, atmospheric pressure plasma may meet a problem of unstable discharge, i.e., plasma often appears at certain local areas. Thus, expensive cyanide gas or helium must be used to serve as an activator.
Because of the above problems, plasma being applied in refrigerators is still difficult in technology. Atmospheric pressure plasma with low cost and high efficiency is an issue to be solved by the industry.